The present invention is directed to a method of and apparatus for operating a focused microwave sample digestion system such that, once set up, human intervention is not required. More particularly, the present invention is directed to a method of and apparatus for automatically operating a focused microwave sample digestion system in which a plurality of prepared samples are moved through a focused microwave digestion system and then placed in a storage rack after each of the samples has been processed, all in an automatic manner, without the need for human intervention.
During the process of testing for the presence of trace metals in certain organic samples, such as tobacco or paper samples, it is necessary to destroy all of the organic material contained in the sample. Such destruction is typically performed by adding an acid, such as concentrated nitric acid, to a vessel containing the sample and then cooking or digesting the sample at a very high temperature.
Typically, the sample and nitric acid combination is digested in a microwave oven, in order to prepare the sample for trace analysis by an inductively coupled plasma optical emission instrument. Recently, however, focused microwave-based systems have become available which allow speeded-up digestion times. One such microwave digestion oven is the CEM STAR 6 Focused Microwave System, which is available from CEM, Inc., of Matthews, N.C. Using the STAR 6 Focused Microwave System, it is possible to digest a sample in approximately 15 minutes, with six samples being digested at one time. Thus, for example, in order to digest 24 samples, one hour is required. Additional time is necessary, however, to load and unload the samples. Using prior microwave technology, it takes about 4.5 to 5.0 hours to digest 24 samples.
However, even though using the STAR 6 Focused Microwave System speeds up the cooking process, it still requires the full attention of a lab analyst who must load the sample tubes into the microwave oven system, start the microwave oven system, and monitor the system while it is digesting samples. It would, however, be advantageous to perform the digestion process and loading and unloading operations on an automated basis such that lab personnel would be freed up for performing other tasks. In addition, if the digesting process were automated, it could be operated unattended after business hours, which would likewise serve to increase the sample capacity throughput. That would allow the laboratory utilizing the automated system to keep pace with increased testing demands. In addition, more uniform results could be obtained if the steps involved in performing the acid digestion were uniformly applied under computer control, rather than being performed on an as-needed basis by a laboratory technician.